Process for prilling urea

ABSTRACT

Urea is prilled by spraying urea droplets downwardly into a solidification zone where it is contacted with a substantially inert gas. Prills are directed into a collector by diverting their flow by directing a gas through a perforated, inverted cone or pyramid located at the bottom of and in communication with said zone.

RELATED APPLICATIONS:

This application is a continuation of application Ser. No. 255,760,filed May 22, 1972 (now U.S. Pat. No. 3,836,611) which, in turn, is acontinuation-in-part of abandoned application Ser. No. 104,998, filedJan. 8, 1971. The instant application is also related to applicationSer. No. 255,726, filed May 22, 1972, which has matured into U.S Pat.No. 3,819,310.

FIELD OF INVENTION

This invention is related to the processing of an aqueous urea solutionto anhydrous solid urea of the shape of small spherical particlesusually called urea prills.

BACKGROUND OF THE INVENTION

Urea is commercially obtained by reacting NH₃ and CO₂ at elevatedpressure and temperature to form ammonium carbamate and tosimultaneously dehydrate ammonium carbamate to urea. The aqueous ureasolution produced in a urea synthesis plant usually contains about onemole of water per mole of urea formed in the urea synthesis reactor. Theaqueous urea product solution (generally 75-76 weight percent) isusually processed to a solid anhydrous form by means of the followingsequential steps:

1. Practically all of the water contained in the aqueous urea productsolution is evaporated to form a more or less pure urea melt;

2. The resulting urea melt at about 270°-300°F. is finely divided intosmall droplets either by means of a spray head, a spinning conicalbasket, or a vibrating plate, such devices usually being located at thetop of a tall, vertical, cylindrical, rectangular or square tower100-150 feet high;

3. The molten urea droplets are allowed to fall freely inside the towercountercurrently to an uprising stream of ambient air;

4. The finely divided urea droplets in their free fall inside the towerare cooled by the uprising stream of ambient air and they are frozeninto the shape of small spherical particles (prills);

5. The frozen prills are usually cooled further to about 100°-150°F. andare collected at the bottom of the tower.

Alternate techniques can be used to produce molten urea in step 1described above. One is the crystal-melting technique, which consists ofcrystallizing the aqueous urea product solution to produce pure crystalurea, which is washed, dried and remelted to produce a substantiallypure urea melt.

Referring to the step 5 described above, various methods of collectingsolid urea prills at the bottom of the prilling tower have been proposedaccording to the prior art. In general these methods, mainly listedbelow, have certain specific disadvantages.

One method consists of collecting the solid urea prills by means of aprilling tower bottom of the shape of a truncated inverted cone orpyramid, with steep, flat sides with an opening at the bottom apex ofthe inverted cone, through which the prilled urea product is withdrawn.This type of prill collecting bottom has the drawback of frequent solidurea build up on the conical sides and on the bottom section of theinverted cone, with consequent plant shut downs for cleaning operation.Another drawback of this type of prill collecting bottom is the factthat relatively taller and thus more expensive prilling towers arerequired due to the very steep angle with the horizontal, 60° or more,at which the sides of the inverted cone must be designed in order tofacilitate the sliding of the solid prilled urea product towards thebottom opening.

Still another commonly used method consists of collecting the solidprilled urea product by means of a flat horizontal prill tower bottom,which is provided with a collecting rake travelling in a circular motionand thus pushing the solid prilled urea product toward the center of thehorizontal prill tower bottom for discharge onto a belt conveyor. Such aprilling tower usually has the drawback of solid urea build up on theprilling tower bottom and on the collecting rake, and the drawback of arelatively higher product degradation due to attrition with a consequentgreater air pollution problem in the subsequent solid handling steps.

A third method commonly used in the industry consists of collecting thesolid prilled urea product by means of a horizontal fluidized bed ofsolid urea prills of several inches in depth, which is maintainedfluidized by blowing a substantial amount of air through a perforatedhorizontal metallic surface upwardly into the prilling tower. The excesssolid prilled urea product collected in the bottom fluidized bed isoverflowed from the fluidized bed over a weir and into a collectingtrough. Such a method has the drawback of the instability of theoperation due to the collapsing of the fluidized bed of urea prills atthe slightest variation in air flow through the perforated horizontalsurface. Another drawback of this method is the relatively highelectrical power consumption by the large air blowers required tomaintain the bed of solid urea prills properly fluidized throughout thefull horizontal cross sectional area of the prilling tower bottomsection.

For example, in U.S. Pat. No. 3,615,142 (Dahlbom), a prilling tower withan inverted frustum trough bottom exit is described. A louverconstruction is employed in the trough for the purpose of air beingpassed through the louvers to cool the prills falling through the tower.This construction has the disadvantage of offering relatively largesurface areas of the louvers on which prills can collect. With alouvered construction, there is a substantial build up of prills uponthe individual louvers and large dead air areas. With this build up ofprills, it is difficult to remove the prills from the bottom section ofthe tower. With excessive prill build up on the louvers, air flow isseriously reduced; then, hot soft prills fall to the bottom andaggregate and cause physical failure of the louvers. Further, the air isnot finely dispersed passing through the louvers.

In U.S. Pat. No. 3,457,336 (Harris), urea or other droplets of moltenmaterial are passed through a zone containing a dust bearing gas inorder to obtain substantially spherical granules. In all of thearrangements described, a primary source of air is introduced directlyinto the bottom exit of the system. Secondary, and even tertiary, airsources are employed, particularly to maintain a fluidized bed of dustparticles.

With the primary air introduced through the prill exit, there is nocontrol of the falling prills and they tend to build up upon the wallsof the tower directly above or in the vicinity of the exit. Then, as theprills stick to the tower walls, they plug the tower completely abovethe exit and the tower must be taken out of service for cleaning.

A fluidized bed requires a tremendous amount of power for the fluidizingair flow required to maintain such a bed. Fluctuation of air supply willlead to collapse of the bed.

It has been found that by operating a prilling tower bottom according tothe process described further below, all the drawbacks which areinherent to the prior art described above are greatly reduced, if notcompletely eliminated.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a processfor prilling urea and other substances. With regard to urea, moltensubstantially anhydrous urea is sprayed and falls freely downwardlythrough a solidification zone in the form of droplets which arecontacted with a substantially inert and relatively cooler gas passedinto said zone. The droplets are cooled thereby and are solidified assubstantially anhydrous prills. The prills are collected in a collectingtrough located at the bottom of the solidification zone. A gas is passedthrough the collecting trough countercurrently to the downwardly flowingprills, the collecting trough comprising an inverted cone or pyramid theapex of which defines an open discharge zone and the sides of which areperforated and oblique and which allow said gas to pass therethrough asa fine dispersion to the solidification zone. The amount of gas issufficient to divert the downwardly flowing prills from theirsubstantially vertical path to the discharge zone to substantiallyprevent the downwardly flowing prills from contacting the perforatedoblique sides, and to avoid build-up of the prills upon said perforatedoblique sides.

DRAWING

The FIGURE is an elevational view of a prilling tower illustrative ofthe invention.

SPECIFIC EMBODIMENT OF THE INVENTION

Referring to the drawing, stream 1 containing substantially molten ureaat about 280°F. is passed through spray heads 2, located in top section3 of prilling tower 4. The molten urea sprayed into a stream of smallliquid droplets through spray heads 2 is allowed to fall freely insidethe prilling tower 4 countercurrently to an uprising stream of ambientair introduced into the prilling tower at its bottom section 5. Thisambient air, introduced into the prilling tower through lines 6 and 7,in its upwardly flow picks up heat from the molten urea whichcrystallizes in the shape of small spherical particles (prills). The hotsolid urea prills are further cooled to about 10°-20°F. above theambient temperature of the air in lines 6 and 7 during their free fallinside the tower 4.

The ambient air introduced into the prilling tower 4 is heated up by theequivalent amount of heat released by the urea prills during their freefall, and it is discharged from the prilling tower 4 through lines 8located in or near top section 3.

The bottom section 5 of prilling tower 4 is provided with a hollowcollecting trough 9 in the form of an inverted cone or frustum withsides 10 made of suitable material as concrete or metal. The sides 10 ofthe inverted trough 9 are solid and are at an approximate angle of about20°-30° with the horizontal, and they are covered from the inside withfine wire mesh 11, which is held in place by means of spacers 12. Thewire mesh is provided to disperse the air which is passed therethrough;it has little surface area such that product cannot collect thereon. Thesurface area of the solid portion of the mesh is substantially smallerthan the surface area of the openings therein. The dead air areas of themesh are minimal. Spacers 12 are positioned in such a manner as tomaintain the wire mesh cover 11 at an approximate distance of about 1-2inches from the sides 10.

Free space 13 between the wire mesh 11 and the sides 10 is sealed at itsends 14 and 15, respectively, so as to form a sealed chamber all aroundthe total surface of the sides 10. Further, vertical walls 16 and flatbottom 17 of the bottom section 5 of prilling tower 4 form a sealedchamber 18 with the sides 10 of the inverted trough 9. As shown in thispreferred embodiment, a minor portion, from about 5 to about 30 percentby volume, of the total amount of ambient air fed to the prilling tower4 is passed into sealed chamber 18 through line 7 and thence throughopenings 19 provided on the sides 10 into space 13 between the wire mesh11 and the sides 10. It is to be understood that all of the air employedcan be fed through line 7. The ambient air of line 7 is evenlydistributed along the whole surface of the trough 9 by being passedthrough the wire mesh 11 and is finely dispersed as it passes throughthe wire mesh. As it leaves the wire mesh surface, the air is sufficientto divert the free falling solid urea prills from their vertical path toan oblique path which is converging toward bottom opening 20, located atthe apex of the inverted trough 9. Thus, substantially all of the prillsdo not impinge upon mesh 11, but are diverted toward bottom 20. Thisprevents undesirable build up of the prills on the surface of mesh 11,which would impede upward air flow and reduce the efficiency of toweroperation. Due to the fact that there is practically no upward air flowin the section just above the bottom opening 20, the solid urea prilledproduct falls through the bottom opening 20 onto belt conveyor 21, fromwhich it is discharged at 22 ready for storage or bagging. As indicated,air is not introduced into bottom opening 20 countercurrent to theproduct passing therethrough. A rubber or other flexible skirt (notshown) can be located around the bottom opening 20 to seal the bottomproduct discharge area, and to minimize leakage of air fromsolididification zone 23.

Urea dust present in the tower is removed through bottom opening 20 andcan be separated from urea prills as undersized material in a commercialshaker or the like (not shown). In contrast to prior art towers, thedust is not blown upwardly and out of the tower. Thus, pollutionproblems are minimized with the method of this invention.

In this preferred embodiment, a major portion, from about 95 to about 70percent by volume, of the ambient air fed to the prilling tower 4 isintroduced into the bottom section 5 through lines 6 located above andpreferably just above the inverted trough 9. This air is mixed with thesmaller portion of the ambient air introduced into the prilling tower 4through line 7; the total mixture of the two streams of ambient airrises through the prilling tower 4 and it is exhausted from the topsection 3 of the prilling tower 4 through lines 8.

Since the urea droplets are solidified in tower 4, the portion of thetower below spray heads 2 and above trough 9 is considered to comprisesolidification zone 23.

In another embodiment, the major portion of ambient air can beintroduced into the upper section 3 of the prilling tower 4 through thelines 8, passed downwardly and co-currently to the stream of freefalling product and exhausted from the prilling tower 4 through thelines 6 on the bottom section 5 of the prilling tower 4 together withthe minor portion of air introduced into the prill collecting invertedtrough 9 through line 7.

EXAMPLE

Referring to the FIGURE, a stream of 25,000 lbs/hr of molten urea, atabout 280°F., containing about 0.2 weight percent of water and about 0.3weight percent of biuret, is passed through line 1 and is sprayedthrough a system of spray heads 2 inside a prilling tower 4 having acircular cross-section about 25 feet in diameter. The fine droplets ofmolten urea thus formed are allowed to fall freely inside the prillingtower 4 for its full height of about 140 feet countercurrently to astream of uprising air, which is exhausted at about 95°F. to theatmosphere through lines 8 located in the upper section 3 of theprilling tower 4.

The total amount of air exhausted to the atmosphere through the lines 8is an amount of about 200,000 standard cubic feet per minute (scfm), ofwhich amount about 170,000 scfm is introduced into the bottom section 5of the prilling tower 4 through lines 6 at about 70°F.

By the time the free falling urea particles reach the bottom section 5of prilling tower 4 they are solidified into the shape of small spheresof about 1.5 mm is diameter and are cooled to about 100°F.

About 30,000 scfm of ambient air at about 70°F. is introduced into thesealed chamber 18 of the prill collecting inverted trough 9; throughopenings 19, the air is passed through a 50 mesh wire cloth 11 coveringthe sides 10. This amount of ambient air forced through the wire mesh 11at a relatively high velocity diverts the free falling urea prills fromtheir vertical path into an oblique path converging toward the bottomcircular opening (circular) 20 of about 3 feet in diameter located inthe center of the tower cross-section.

The solid urea prills at about 100°F. are discharged through bottomopening 20 onto belt conveyor 21 and are delivered to storage or baggingat 22. Stream 22 consists of 25,000 lbs/hr of solid prilled ureacontaining about 0.2 weight percent of water and about 0.3 weightpercent of biuret.

Air is shown as a suitable gas for cooling and diverting free fallingsolid material in tower 4. It is to be understood that any gassubstantially inert to the solid material, as urea, can be employed. Inaddition to dry or humid air, or in admixture therewith, there can beused: nitrogen and carbon dioxide. SO₂ and SO₃ can also be employed withurea, some sulfur or sulfur compound forming on urea prills as a coatingtherefor. Steam is not suitable, since it would increase the moisturecontent in the prilled product.

Instead of forming urea prills, the process can be used to form andcollect such fertilizer substances as ammonium nitrate, ammoniumphosphate, calcium nitrate, calcium phosphate, potassium chloride, etc.,from molten streams thereof charged to tower 4 from line 1. A mixture oftwo or more of such substances, including urea, can be so chargedthrough line 1.

Mesh 11 can be replaced by other suitable elements which provide desiredfine air dispersions. A substitute can be, for example, a perforatedplate preferably having a total opening area larger than plate surfacearea.

I claim:
 1. In a process for prilling urea, comprising:a. sprayingmolten, substantially anhydrous urea downwardly as droplets in aprilling tower through a solidification zone therein generally towardthe bottom of said tower; b. contacting said droplets with asubstantially inert gas passed into said zone to cool and solidify saiddroplets as anhydrous prills; and c. collecting said prills at acentrally located bottom of said zone;the improvement comprising: d.passing said gas through a hollow, perforated, inverted frustum troughcountercurrently to said downwardly falling prills and maintaining gasflow, gas velocity and the angle of said trough through which said gasis so passed as a fine dispersion, so as to divert said downwardlyflowing prills from a substantially vertical path to an oblique pathtoward said bottom of said zone and to prevent substantially all of saidprills from contacting said trough.
 2. The process of claim 1, whereinsaid inert gas is air.
 3. In a process for producing substantiallyspherical fertilizer particles, comprising:a. spraying molten,substantially anhydrous fertilizer in finely divided particle formdownwardly in a tower through a solidification zone generally toward thebottom of said tower; b. contacting said particles with a substantiallyinert gas passed into said zone to cool and solidify said particles; andc. collecting the resulting cooled and solidified particles at acentrally located bottom of said zone;the improvement comprising: d.passing said gas through a hollow, perforated, inverted frustum troughcountercurrently to said downwardly falling cooled and solidifiedparticles and maintaining gas flow, gas velocity and the angle of saidtrough through which said gas is so passed as a fine dispersion, so asto divert said downwardly flowing cooled and solidified particles from asubstantially vertical path to an oblique path toward said bottom ofsaid zone and to prevent substantially all of said cooled and solidifiedparticles from contacting said trough.
 4. The process of claim 3,wherein said inert gas is air.